1. Field of the Invention
The present invention relates to a torsion spring for a micro electro mechanical system (MEMS) structure. More particularly, the present invention relates to a torsion spring having a greater bending stiffness than a torsional stiffness.
2. Description of the Prior Art
Micro electro mechanical system (MEMS) technology is used in the manufacture of mechanical and electric elements through semiconductor processes. MEMS technology usually includes a structure mounted on a substrate and other parts, such as a spring, to support the structure and allow the structure to move in a predetermined direction. For example, to manufacture a MEMS gyroscope, there should be a mass mounted on a substrate and a torsion spring formed on the substrate in order to support the mass, which is to be rotated around a particular axis.
In the above MEMS structure, the torsion spring should allow the structure to be rotated in a certain rotational direction and limit the rotation of the structure in other directions. For the torsion spring to operate effectively, the torsion spring should have a high bending stiffness, i.e., the stiffness of torsion in a direction perpendicular to an axis of torsion, and a low torsional stiffness, i.e., the stiffness of torsion in a direction parallel to the axis of torsion.
A torsion spring used in a macro structure should have a circular section or a cross-shaped section so that a bending stiffness is greater than a torsional stiffness. However, in a MEMS structure, the manufacturing process may be very complex or require several additional processes to manufacture a torsion spring having a circular section or a cross-shaped section as described above.
A conventional torsion spring for a MEMS structure is fabricated as a beam with a quadrilateral section, as shown in FIG. 1. In the torsion spring 10 of FIG. 1, the bending stiffness and the torsional stiffness are determined based on a ratio of the width, length, and height of the beam. For example, as the length of the beam increases, both the bending stiffness and the torsional stiffness become weaker. Therefore, it is difficult to manufacture a torsion spring, constructed as shown in FIG. 1, having a high bending stiffness to torsional stiffness ratio.
To solve the above problem, a conventional torsion spring 20, as shown in FIGS. 2A and 2B, has been proposed. The conventional torsion spring 20 includes a connection plate 23 to connect upper parts of a pair of beams 21, the connection plate 23 being disposed perpendicular to the pair of beams 21 and extending the length of the beams 21. Thus, in the above structure, the bending stiffness may be increased without significantly increasing the torsional stiffness. The torsion spring 20 of FIGS. 2A and 2B, however, has a disadvantage in that it requires several additional processing steps. Moreover, the torsion spring 20 cannot be fabricated by a single etching process.